[h=1]Why do trolls annoy you online? Because they're sadists.[/h]
Though not just sadists â€” researchers say online trolls also score above average in narcissism, Machiavellianism, and psychopathy.
In a new study published in Personality and Individual Differences, researchers from the University of Manitoba looked at the correlation between internet trolling and the four traits that are part of the (alarming sounding) "Dark Tetrad." The result? Higher scores in all four among internet trolls. Chris Mooney, over at Mother Jones, explains:

It is hard to underplay the results: The study found correlations, sometimes quite significant, between these traits and trolling behavior. What's more, it also found a relationship between all Dark Tetrad traits (except for narcissism) and the overall time that an individual spent, per day, commenting on the Internet . . . They even constructed their own survey instrument, which they dubbed the "Global Assessment of Internet Trolling" or GAIT, comprised of the following items:"I have sent people to shock websites for the lulz." "I like to troll people in forums or the comments section of websites." "I enjoy griefing other players in multiplayer games." "The more beautiful and pure a thing is, the more satisfying it is to corrupt."
Yes, some people actually say they agree with such statements. And again, doing so was correlated with sadism in its various forms, with psychopathy, and with Machiavellianism. Overall, the authors found that the relationship between sadism and trolling was the strongest, and that indeed, sadists appear to troll because they find it pleasurable. "Both trolls and sadists feel sadistic glee at the distress of others," they wrote. "Sadists just want to have fun...and the Internet is their playground!"
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Researchers also made this graph of how strong the representation of each trait was among trolls compared with other groups:
Of course, trolls form a fairly small percentage of the online population â€” just over 5%, and that's only out of people who comment. Researchers also note that there's a fairly significant minority of readers, over 40%, who only read things online and never comment.
You can read more about the research over at Mother Jones, or check out the whole study here.

I think they're onto something here. One reason being â€” warning: anecdotal evidence to follow â€” seems to me that I've noticed trollish attacks seem to hit posts that seem mild-mannered rather than aggressive, and I attribute this to the presumed troll going after "prey" they see as weaker. Yesterday 9:29am

My favorite are the studies that determine certain human pathologies have both a biological & socialization component. SHOCKING.

As someone who once majored in it, what a worthless fucking discipline. You do more teaching humanity to observe & make connections versus cold statistics & data that people whip out with no added insight.